Deposition of pyrolytic material



United States Patent 3,462,522 DEPOSITION OF PYROLYTIC MATERIAL ThomasJ. Clark, Troy, and Bruce L. Ettinger, Washington, Mich., assignors toGeneral Electric Company, a

corporation of New York No Drawing. Filed Dec. 2, 1966, Ser. No. 598,633

Int. Cl. B29c 13/04; B28d 7/38; C01b 31/00 US. Cl. 264-39 10 ClaimsABSTRACT OF THE DISCLOSURE Pyrolytic articles are deposited on a mandrelsurface by initially oxidizing the surface with an oxidizing gas,heating the mandrel and depositing by vapor deposition a very thinpre-coat of pyrolytic material over the oxidized surface, interruptingthe deposition and finally depositing by vapor deposition the desiredpyrolytic article.

Background of the invention This invention relates to a method andapparatus for forming graphite articles by pyrolysis of a carbonaceousgas, and more particularly to an improved mandrel on which to depositpyrolytic graphite and to a method for preparing same.

Pyrolytic graphite because of its extremely high temperature resistanceand its nuclear and other desirable properties has a broad field ofutility such, for example, as for lamp filaments, furnace linings,nuclear reactor moderators, rocket nozzles and re-entry heat shields.Pyrolytic graphite is manufactured by pyrolysis, or thermaldecomposition, of a carbonaceous gas. Any of a wide variety ofcarbonaceous gases may be used, though in practice methane, either aloneor in combination with hydrogen, is preferred. Manufacture generallyrequires that the pyrolytic graphite article be of tubular shape orcircular crosssection. To make a tube of pyrolytic graphitethecarbonaceous gas is passed through a tubular mandrel, the mandrelbeing a a sufliciently high temperature to cause pyrolysis of thecarbonaceous gas. This pyrolysis results in the deposition of thepyrolytic graphite on the interior wall of the tubular mandrel. Thegraphite deposits in laminae and the process is continued until thedesired thickness is accomplished. The mandrel having the depositedarticle thereon is then cooled. Finally, the formed article is separatedfrom the mandrel.

Since the deposition of the pyrolytic graphite is generally carried outat temperatures from 1200 C. to 2500 C., preferably from 1800 C. to 2300C., the mandrels used in the process must be able to withstand suchelevated temperatures for extended periods of time. Ordinarily thesemandrels are fabricated from electrographite. This material not only hasthe necessary high temperature resistance and a satisfactory coeflicientof thermal expansion but is also relatively inexpensive, which meansthat the mandrel can be broken and destroyed, for purposes of obtaininga good separation of the pyrolytic article from the mandrel, withoutserious cost disadvantage.

Electrographite itself is made from a mixture of coke particles and apitch binder by baking the mixture at about 750 C. to 1200 C. and thengraphitizing it at about 2500 C. The resultant electrographite ispolycrystalline, and contains numerous voids composed mostly of tortuousinner-connected pores between the various graphitized coke particleswhich are bonded together by the graphitized pitch. These voids explainwhy electrographite weighs only about from 70% to 80% of its theoreticaldensity. Moreover, because of these voids, when electrographite ismachined into tubular mandrel form, its inte- 3,462,522 Patented Aug.19, 1969 rior wall, upon which the pyrolytic graphite article will bedeposited, is not continuous, but rather consists of machined cokeparticles separated by pores.

The texture of the machined coke particles depends upon the local randomorientation of the cleavage planes of the crystallites that are presentin the coke particles. If, by chance, the coke particle is machinedalong a cleavage plane, the surface will be relatively smooth; however,as is much more probable, the machining will take place at some angle tothe local cleavage planes which results in a relatively irregularsurface.

These surface irregularities can affect the structure of the depositedarticle and such irregularities frequently generate large isolatednodules in the deposited article. Such nodules may act as local stressraisers and failure of the deposited article has often been found to beassociated with their existence. In order to provide a more uniformlytextured mandrel surface than machined electrographite, it has beenproposed to deposit on the interior wall of the mandrel a relativelythick pre-coat of pyrolytic graphite. However, the use of such apyrolytic graphite pre-coat has not proved to be satisfactory sincefrequently part of this pre-coat sticks to the deposited article uponbreakage and separation of the mandrel from the article, causing thearticle to have a spalled surface and often to be out of tolerances.Such defects generally cannot be corrected by machining since machiningof the deposited article is frequently detrimental to the properties inother respects. Hence the art is in need of an improved mandrel for themanufacture of pyrolytic graphite, and a method for making suchmandrels. The present invention fulfills this need.

Summary of the invention Briefly, what we have found is that a mildoxidation treatment carried out by passing an oxidizing gas, such asoxygen or carbon dioxide, through a tubular electrographite mandrelbefore the pyrolytic article is deposited results in the interior wallof the mandrel being far better adapted to receive the deposition of apyrolytic graphite article than it was prior to the oxidation treatment.This mild oxidation treatment leads to an improved mandrel release afterdeposition, superior surface smoothness of the deposited article, andminimizes the formation of large isolated nodules in the depositedarticle. In addition, we have discovered that these features can be evenfurther enhanced by depositing a very thin precoat of pyrolyticgraphite, i.e., a layer less than .002 inch in thickness, over theoxidized interior wall of the mandrel. Hence, in this preferredembodiment, the mandrel consists of the oxidation-treatedelectrographite surface With the thin pre-coat of pyrolytic graphitethereover.

Description of the preferred embodiment The mild oxidation treatment isaccomplished by passing the oxidizing gas, such as carbon dioxide oroxygen (either as such or as air), over the surface of theelectrographite mandrel while the mandrel is at a temperature suflicientto cause the desired oxidation reaction, generally from about 400 C. to1300" C. The precise temperature used will depend on the oxidizing gasemployed. For example, with a relatively mild oxidizing gas such ascarbon dioxide, a relatively high temperature can be used, on the orderof 800 C. to 1300 C., whereas with a stronger oxidizing gas such asoxygen, 21 lower temperature, from 400 C. to 800 C., is desirable. Theoxidation should be halted before the mandrel suffers more than about a1% weight loss, since continued oxidation could result in a weakenedmandrel due to oxidation of the graphitized binder. This oxidationtreatment is preferably carried out immediately prior to the deposi- 3tion of the pyrolytic graphite, be it the thin pre-coat in accordancewith the preferred embodiment of the invention or, where the thinpre-coat is not used, the pyrolytic graphite article desired to bemanufactured. Otherwise damage of the prepared surface may occur byaccidental contact during handling or storage.

At the completion of the oxidation treatment the temperature of themandrel is, of course, raised to that desired for the pyrolytic graphitedeposition, be it the pre-coat or, where the pre-coat is not used, thearticle desired to be manufactured. Where the pre-coat is used, at thecompletion of the desired pre-coat depositions, the flow of carbonaceousgas should be interrupted prior to commencement of the deposition of thepyrolytic graphite article of manufacture. The temperature used fordeposition of the article may be different than that used for thedeposition of the pre-coat.

As has been alluded to above, the mild oxidation treatment will resultin easier release of the mandrel from the deposited article and superiorsurface smoothness of the deposited article. In addition, and mostsignificantly, the formation of large isolated nodules in the depositedarticle is minimized.

Although the precise reasons for the improvements are not fully known,we theorize that oxidation occurs on the mandrel surface more rapidly atexposed crystallite edges than at crystallite faces when such edges andfaces are exposed, as by machining. This result would theoreticallyoccur owing to the different nature of the chemical bonding in the twodirections of the anisotropic crystallites which make up the graphitizedcoke particles. It is probably as a result of this same structuralanisotropy that cleavage occurs between the planes more readily thanacross the planes of the carbon atoms during machining with the resultthat the crystallite edges form hill-like structures at the exposedsurface. Upon oxidation, these crystallite edges are oxidized morerapidly than the faces, resulting in a more uniformly textured surfacethan that of an untreated machined surface of electrographite.

Moreover, as already indicated, an even more desirable surface willresult from the addition of a thin pre-coat, less than .002 inch, ofpyrolytic graphite on the treated surface. This thin layer is depositedby passing through the mandrel a measured amount of carbonaceous gas,such that the resulting pyrolytic graphite deposit on the inner tubularwall of the mandrel does not exceed .002 inch. This thin layer fills thepores of the treated electrographite surface and, in addition, providesfilleting which results in a smoother surface than that achieved by themild oxidation treatment alone. After the thin pyrolytic pre-coat isdeposited, the pyrolytic graphite article is then deposited in the usualmanner. Upon separation of the mandrel from the desired depositedpyrolitic graphite article, even if part of the thin layer sticks to thedeposited article, its thickness is so minute as to be negligible.

Although it is much preferred if the thin pre-coat, less than .002 inch,of pyrolytic graphite is used in combination with a mandrel surfacewhich has been treated by the mild oxidation treatment, we have foundthat, even used alone, such a thin pre-coat not only serves as a goodsurface for receiving the pyrolytic graphite article but also enablesmaintenance of good surface finish and close tolerances in the pyrolyticgraphite article of manufacture since even if portions of the thin layeradhere to the article, the effect on dimensions is negligible.

Also, whereas the invention has been taught specifically with referenceto the deposition of pyrolytic graphite both for the pre-coat and forthe article of manufacture, it will be understood that other pyrolyticmaterial can be used for both or either the pre-coat and the article.For example, the pre-coat can be pyrolytic graphite and the articledeposited on the pre-coat can be an alloy of pyrolytic graphite such asboron alloy of pyrolytic graphite. As a further example, the depositedarticle of manufacture can be boron nitride, with the pre-coat beingeither pyrolytic graphite or boron nitride.

What is claimed is:

1. A method for manufacturing an article of pyrolytic material by thepyrolytic deposition from a gas on an electrographite mandrel comprisingsubjecting the surface of said mandrel to an oxidizing gas to causeoxidation of surface portions thereof, pyrolytically depositing on saidsurface portions a pre-coat of pyrolytic material having a thickness ofless than .002 inch, interrupting the pyrolytic deposition, and thenpyrolytically depositing said article on said pre-coat.

2. The method as defined by claim 1 wherein the quantity of oxidizinggas utilized is insufficient to oxidize more than 1% by weight of saidmandrel.

3. The method as defined by claim 1 wherein said material of saidarticle and said pre-coat is graphite, and wherein the gas iscarbonaceous.

4. A method for manufacturing an article of pyrolytic material by thepyrolytic deposition of a gas on an electrographite mandrel comprisingsubjecting the surface of said mandrel to an oxidizing gas to causeoxidation of surface portions thereof, and then pyrolytically depositingsaid article on said surface portions.

5. The method as defined by claim 4 wherein the quantity of oxidizinggas utilized is insufficient to oxidize more than 1% by weight of saidmandrel.

6. The method as defined in claim 4 wherein said material is graphiteand where the gas is carbonaceous.

7. A method for manufacturing an electrographite mandrel having asurface adapted to receive a pyrolytic article comprising subjecting thesurface of said mandrel to an oxidizing gas to cause oxidation ofsurface portions thereof and thereby impart thereto a uniform texturedsurface.

8. The method as defined by claim 7 additionally including the step ofpyrolytically depositing on said surface a layer of pyrolytic graphitehaving a thickness of less than .002 inch.

9. A mandrel adapted for the pyrolytic deposition of a pyrolyticmaterial including an electrographite member, surface portions of saidmember having been treated with an oxidizing gas to cause oxidation ofsaid surface portions and thereby impart thereto a uniform texturedsurface.

10. The mandrel as defined by claim 9 additionally including a layer ofpyrolytic material having a thickness of less than .002 inch depositedon said oxidized surface portions of said mandrel.

References Cited UNITED STATES PATENTS 2,201,049 5/1940 Moore 2491l5 XR3,206,331 9/1965 Diefendorf 117-146 XR 3,367,826 2/1968 Heestand et a1.117--46 XR DAVID KLEIN, Primary Examiner US. Cl. X.R.

